The long-term goal of this project is the study of the fundamental biological processes which control the inheritance, growth and replication of mitochondria in mammalian cells. These studies should also provide information about the interrelationship between the nucleus and mitochondria, as well as answers to questions about the regulation of mitochondrial genome expression. We will examine interspecific somatic cell hybrids (mouse X hamster) that stably retain the mitochondrial DNA of both parents. These hybrids synthesize only mouse mitochondrially coded proteins and normal amounts of mouse mitochondrial ribosomal and messenger RNA. No hamster proteins can be detected. Experiments will be described that examine the transcription and translation in hybrid cell mitochondria both in vitro and in vivo in order to determine the factor(s) that are involved in the regulation of expression of the mitochondrial genome.